Portuguese Man o’ War
Sometimes the most common indication that someone is drowning is that they don’t look like they’re drowning. They may just look like they are treading water and looking up at the deck. One way to be sure? Ask them, “Are you all right?” If they can answer at all—they probably are. If they return a blank stare, you may have less than 30 seconds to get to them. And parents—children playing in the water make noise. When they get quiet, you get to them and find out why.
Via Rescuing drowning children: How to know when someone is in trouble in the water. – Slate Magazine.